1. Field of the Invention
The present invention relates to a predistorter and particularly, to the predistorter which has a variable gain circuit for power control.
2. Description of the Prior Art
A predistorter is conventionally used for the purpose of preliminarily compensating, at a baseband, a distortion caused in a transmission power amplifier of a radio communication apparatus, and thereby reducing a spurious radiation caused by a non-linearity of the transmission power amplifier. In the case where a predistorter is used in the radio communication apparatus, a transmission power amplifier can operate in a non-linear region and accordingly, the power consumption during transmission of the radio communication apparatus can be decreased. If the radio communication apparatus is a portable phone which uses a battery as its power source in such a case, a talk time can be elongated.
Predistorters as explained above have been developed actively heretofore. The theory and structure of a predistorter is explained in detail in xe2x80x9cQuantization Analysis and Design of a Digital Predistortion Linearizer for RF Power Amplifierxe2x80x9d, Sundstrom L., Faulkner M., Johansen M.; Vehicular Technology, IEEE Trans. Vol. 45 4, pages 707-719.
A conventional predistorter will be explained hereunder while making reference to the above article.
In conventional mobile communications, it has been adopted a constant envelope modulation system such as FM system and GMSK system, in which an amplitude of a modulated signal is constant and information is carried on phase variation of the modulated signal. The reason for the adoption is that the constant amplitude enables the use of a C- or F-class non-linear amplifier which has a high power efficiency as a transmission power amplifier. In the case where a transmission power amplifier with a high power efficiency is used, the power consumption during a transmission is reduced and, if a portable phone is used, a talk time can be elongated.
Heretofore, however, increase in traffic has become a problem in mobile communication.
Hence, a modulation system with a high efficiency in frequency use also has been used. For example, PDC (personal Digital Cellular) and PHS (Personal Handyphone System) in Japan adopt xcfx80/4 shift QPSK. In addition, CDMA (Code Division Multiple Access) system adopts QPSK or offset QPSK.
These modulation systems are referred to as linear modulation systems, in which information is carried on amplitude as well as on phase. In such systems, a non-linear amplifier can not be adopted because the amplitude of a modulated signal varies within wide range. That is, it is needed to adopt an amplifier with a linearity as precise as possible.
FIG. 3 shows an example of an output spectrum in the case here an amplifier with a poor linearity is used.
A bold line in FIG. 3 represents an original spectrum in a linear modulation system. In the case where a modulated signal passes through an amplifier with a poor linearity, spurious signals with a bandwidth, for example, three times as wide as the original signal (or third order distortion components) represented by a broken line are generated because of the influence of the third order distortion of the amplifier.
In such case where the spurious signals are generated, ACPR (Adjacent Channel Power Ratio) becomes worse and accordingly, efficiency in frequency use is lowered in spite of adoption of a linear modulation system with high efficiency in frequency use. This phenomenon is referred to as Spectrum Re-generation.
In order to avoid this phenomenon, it is necessary to cause an amplifier to operate in a region where it""s linearity is sufficiently precise.
However, the better the linearity of an amplifier is, the more the amplifier consumes a power and the lower the power efficiency thereof becomes.
Linealizers such as predistorter have been developed as means for alleviating such problem. That is, a countermeasure to this problem is to use an amplifier with high power efficiency and poor linearity and a predistorter for compensating the poor linearity, thereby suppressing spurious powers to adjacent channels.
FIG. 4 shows an example of a distortion of a transmission power amplifier.
As shown in FIG. 4, as an input level increases, a gain gradually decreases and input-output characteristics gradually deviate from a straight line and eventually saturate. In addition, phase characteristics deviate from a certain constant from a point close to where the input-output characteristics begin to deviate from the straight line.
An input signal is a complex number which has an amplitude and an angle (or phase) and represented as:
Sr=(Ir+jQr)exp(j2xcfx80fct),
where
Ir: an in-phase component of a baseband signal
Qr: a quadrature component of the baseband signal
fc: carrier frequency.
Here, in the case where Sr is regarded as representing a baseband signal, the exponential part is omitted and the baseband signal is represented as:
Sr=Ir+jQr
In the case where the signal Sr is inputted to an amplifier, the output level becomes |So|xe2x80x2 because of the distortion, though the output level should be |So| originally. In addition, the phase shifts by xcex81. This relation generates the Spectrum Re-generation, which deteriorates ACPR.
A predistorter is effective to avoid the above. In order to cause the amplifier to output the output level So, it is necessary to supply to the amplifier the input Sp which has been preliminarily calculated instead of the input Sr.
FIG. 5 shows a complex plane for explaining the compensation of a distortion carried out by the predistorter.
As shown in FIG. 5, the predistorter needs to generate the signal Sp which has an amplitude made higher than that of signal Sr in consideration of the saturation of the characteristics of the amplifier and has a phase xcex82 in a direction for canceling the phase rotation.
FIG. 6 is a diagram for explaining the formulation of the compensation of the distortion as shown in FIG. 5.
In FIG. 6, G represents the input-output characteristics of the amplifier and F represents the input-output characteristics of a predistorter. Both G and F include phase and are represented with complex numbers.
It is obvious from FIG. 6 that the signal Sp which has been predistorted is represented as:
Sp=F(|Sr|)xc2x7Sr
It is obvious that F is determined by only the amplitude of Sr or the absolute value of Sr. The output So of the amplifier is represented as:
So=G(|Sp|)xc2x7Sp=G{|F(|Sr|)xc2x7Sr|}xc2x7F(|Sr|)xc2x7Sr
In order to establish a linear relationship between So and Sr, it is necessary to satisfy the following equation:
G{|F(|Sr|)xc2x7Sr|}xc2x7F(|Sr|)=C=constant
If an objective value of C, the characteristics G of the amplifier, and the input signal Sr are determined, then the predistortion function F is determined by using the above equation. Here, the function F is represented with a complex number or with a real part Re and an imaginary part Im.
FIG. 7 shows the structure of an example of general adaptive predistorters. Here, an input signal (or a source signal) can be treated as a complex signal which has, in a real part, an in-phase component Ir with respect to a carrier and, in an imaginary part, a quadrature component Qr with respect to the carrier.
This conventional predistorter comprises: look-up table 23 in which a real part Re and imaginary part Im of a predistortion function is stored and which outputs the function value F in response to the amplitude of the input signal used as an address thereof; complex multiplier 20 which multiplies the function value F with the input signal by complex multiplication; digital-to-analog converter 21 which converts the result of the multiplication in the complex multiplier 20 to an analog signal and outputs the analog signal; quadrature modulator 22 which modulates the analog signal supplied from digital-to-analog converter 21 by quadrature modulation and outputs a modulated signal as an RF (radio frequency) signal; non-linear amplifier 11 which amplifies the radio frequency signal and outputs an amplified RF signal; coupler 12 which shunts the amplified RF signal into two RF signals; antenna 13 which transmits one of the signals shunted by coupler 12 as a radio wave; quadrature demodulator 26 which demodulates the other of the signals shunted by coupler 12 by quadrature demodulation and outputs a demodulated signal as a complex baseband signal; analog-to-digital converter 25 which converts the baseband signal to a digital signal and output the digital signal; and adaptation circuit 24 which compares the digital signal outputted from analog-to-digital converter 25 with the input signal and updates look-up table 23 in accordance with the result of the comparison.
In the predistorter having the structure as explained above, the structure of adaptation circuit 24 becomes complicated because adaptation circuit 24 needs to have a function of comparing the demodulated signal and the input signal while adjusting the timing and phase therebetween. In addition, a transmission level from antenna 13 must be determined in accordance with the input signal.
On the other hand, it is necessary to vary transmission power within a range as wide as 80 dB in CDMA system in order to solve the near-far effect.
FIG. 8 shows the structure of another example of general adaptive predistorters.
Referring to FIG. 8, this conventional predistorter comprises: amplitude calculation circuit 15 which calculates an amplitude of an input signal using components Ir and Qr of the input signal, ROM 14 in which a real part Re and imaginary part Im of a predistortion function are stored and which outputs the function value F in response to the amplitude of the input signal calculated in amplitude calculation circuit 15 and used as an address thereof; complex multiplier 20 which consists of four multipliers 1 through 4 and two adders 5 and 6 and multiplies the function F and the input signal by complex multiplication; digital-to-analog converters 7 and 8 which convert the results of the multiplication in complex multiplier 20 to analog signals and output the analog signals; quadrature modulator 9 which modulates the analog signals outputted from digital-to-analog converters 7 and 8 by quadrature modulation and outputs a modulated signal as an RF signal; variable gain amplifier 10 which amplifies the RF signal outputted from quadrature modulator 9 with a gain corresponding to a gain control signal supplied from the external and outputs the amplified RF signal; non-linear amplifier 11 which amplifies the RF signal outputted from variable gain amplifier 10 and outputs the RF signal amplified therein; and antenna 13 which transmits the RF signal outputted from non-linear amplifier 11 as a radio wave.
In the predistorter having the structure as explained above, the amplitude of the input signal is calculated in amplitude calculation circuit 15 using input components Ir and Qr, the value of function F is outputted from ROM 14 using the result of the calculation as an address, and the value of function F thus outputted and the input signal consisting of components Ir and Qr are multiplied each other by complex multiplier 20.
Here, a transmission level from antenna 13 is determined not only by the input signal consisting of components Ir and Qr but also by the gain control signal Gc inputted to variable gain amplifier 10, because variable gain amplifier 10 is inserted between quadrature modulator 9 and non-linear amplifier 11.
In the latter conventional predistorter, there is a disadvantage that it is impossible to compensate a distortion accurately when the gain control signal varies because a transmission level from antenna 13 is determined by an amplitude of the input signal and a gain of variable gain amplifier 10 and the gain of variable gain amplifier 10 varies when the gain control signal varies.
Even if adaptation circuit 24 is introduced to the latter conventional predistorter, it is extremely difficult to stably perform adaptation operation because a predistortion is necessary only in a short period while a gain of variable gain amplifier 10 is high.
In order to overcome the aforementioned disadvantages, the present invention has been made and accordingly, has an object to provide a predistorter which correctly compensates a distortion of a non-linear amplifier even when a gain of a variable gain amplifier varies.
According to a first aspect of the present invention, there is provided a predistorter for preliminarily compensating a distortion caused in a circuit comprising a modulator, a variable gain amplifier, and a non-linear amplifier, the predistorter comprising: an amplitude detector for detecting an amplitude of an input signal; a gain detector for detecting a gain of the variable gain amplifier; a first multiplier for multiplying the amplitude of the input signal with the gain of the variable gain to output a product therebetween; a generator for generating a predistortion function corresponding to the product; and a second multiplier for multiplying the input signal with the predistortion function to output a product therebetween to the modulator.
According to a second aspect of the present invention, there is provided a method for preliminarily compensating a distortion caused in a circuit comprising a modulator, a variable gain amplifier, and a non-linear amplifier, the method comprising steps of: detecting an amplitude of an input signal; detecting a gain of the variable gain amplifier; multiplying the amplitude of the input signal with the gain of the variable gain to output a product therebetween; generating a predistortion function corresponding to the product; and multiplying the input signal with the predistortion function to output a product therebetween to the modulator.
According to a third aspect of the present invention, there is provided a transmitter comprising: the predistorter of the first aspect; a modulator for modulating an output of the predistorter; a variable gain amplifier for amplifying an output of the modulator; a non-linear amplifier for amplifying an output of the variable gain amplifier; and an antenna for transmitting an output of the non-linear amplifier as a radio wave.
According to a fourth aspect of the present invention, there is provided a radio communication apparatus comprising the transmitter of the third aspect.
These and other objects, features and advantages of the present invention will become more apparent in light of the following detailed description of the best mode embodiments thereof, as illustrated in the accompanying drawings.